Polyglutarimides, that is, polymers containing upwards of 10 weight percent of units of the structure ##STR2## wherein R.sub.1 and R.sub.2 are separately H or CH.sub.3, and R.sub.3 is H, C.sub.1 -C.sub.20 alkyl, substituted C.sub.1 -C.sub.20 alkyl, aryl, substituted aryl, aralkyl, substituted aralkyl, or heterocyclic, have been known for some years as polymers with unique properties, both in the neat form and in blends. Recently commercial activity has begun to produce them on a large scale for many uses as plastics in their own right, as modifiers of performance of plastics such as poly(vinyl chloride), and as components of binary and ternary blends. Although improved optical properties are important in certain combinations with other polymers, such as clear packaging applications of blends with poly(vinyl chloride), a high degree of clarity and avoidance of yellowness is most important for the uses of polyglutarimides as a clear plastic or a component of multi-layered composites.
All of the polyglutarimides described above, and especially those of the above structure where R.sub.1, R.sub.2, and R.sub.3 are methyl, exhibit a combination of high service temperature, high modulus, weatherability, clarity, barrier properties, and an ability to accept impact modification that offers commercial opportunities in such fields as automobile components, such as light housing, sun roofs, glazing, and the like, in electronics uses, such as optical disks and optical fibers, medical devices, business equipment, building/construction, in glazing, as molded articles, as monolithic bottles, and in multi-layer composites.
The polyglutarimides are also useful in binary and ternary blends with many thermoplastics, and as additives for certain thermoplastics, especially poly(vinyl chloride) (PVC). Blends with PVC exhibit an improved service temperature and are useful in siding, equipment housing, hot-fill packaging, and the like.
The preferred manufacturing process, which is direct, requires no solvents, produces a polymer of good thermal stability, can be adapted to a continuous process, and has advantages over the several other techniques known to the art, is based on U.S. Pat. No. 4,246,374 ('374), incorporated herein by reference, wherein a polymer with a high concentration of ester groups, such as from a (meth)acrylate polymer, and especially from a polymer of methyl methacrylate, is contacted with ammonia or a primary amine in the molten state in an extruder, and the methanol, primary amine, and by-product secondary and tertiary amines removed by vacuum devolatilization.
Other processes for the preparation of polyglutarimides, such as treatment of a methacrylic ester or methacrylic acid polymer in solution or suspension with a primary amine, may also be utilized to produce a polyglutarimide whose properties may also be improved by the extractant process of the present invention.
Polyglutarimides prepared by the '374 process or other processes may be further treated by the method of U.S. Pat. No. 4,727,117 ('117), also incorporated herein by reference, wherein the content of acid and anhydride groups in the polyglutarimide are substantially reduced by reaction with an alkylating or esterifying agent. Particularly efficient in this respect are alkyl orthoesters, such as tetramethyl orthocarbonate, trimethyl orthoformate, dimethyl carbonate, and the like.
Polymers from both the '374 and '117 technology may occasionally exhibit an undesirable yellow color. Such color may be overcome by toners, which however lower the optical transmission of the processed piece. There has existed a need for a process to improve the color and other optical properties of the product without doing harm to any of the properties of the polyglutarimide, and which could easily and cheaply be applied to the manufacture of the polyglutarimide of improved color.
Further, polyglutarimides which are useful in food-packaging applications, such as in monolithic bottles, multi-layer film and bottles, or in blends with PVC for hot-fill applications, need to exhibit a low level of non-aqueous volatiles and of food-simulant extractables. The former refers to volatiles other than water detected in the polymer prior to or after processing into the final object which are detected by analysis of the volatiles above the polymer, such as by head-space analysis of the pellets in a closed container or of the contents of an empty bottle; in the present context, major contributors are the mixtures of amines utilized or formed during the imidization process. The latter refers to materials extracted from a formed object, such as a monolithic bottle or a PVC/polyglutarimide bottle, by food simulants such as water, ethanol-water mixtures, acetic acid-water mixtures, heptane, and the like.
Further, it is of importance to remove from an additive polymer, such as an additive polymer for PVC, materials which may react with components of the PVC or with chemicals used to stabilize and process PVC. For example, in certain formulations of PVC containing organometallic stabilizers, the blend of the stabilized PVC with a glutarimide additive will exhibit poorer heat stability than the same PVC without the glutarimide additive. To overcome the problem, higher levels of stabilizers may be used, adding to raw material cost, or milder processing conditions may be used, adding to processing cost. A means to improve the thermal stability of glutarimide additives in PVC without additional cost would be desirable.
The art is silent on means to produce very low levels of non-aqueous volatiles in polyglutarimides, such as poly(N-dimethyl dimethylglutarimide), and is silent as to description of such improved polyglutarimides themselves. Experience with processing such polyglutarimides has led to a desirable target of reduction of volatiles (as measured by trimethylamine analysis) to below 30 ppm, more preferably to 3 ppm or below, and most preferably to 0 ppm, that is, undetectable by GLC (gas-liquid chromatography) analysis. Levels to below 30 ppm can be obtained by slow extrusion (long residence time) and careful application of vacuum devolatilization, especially by using a second exposure ("re-pass") of the polyglutarimide, but the process is less economical and can lead to some thermal degradation of the polyglutarimide.
Further, for processing of additives such as processing aids, impact modifiers and heat distortion improvers, such as polyglutarimides, with PVC, it is desirable to decrease food-simulant extractables so as to achieve acceptable organoleptic properties for materials packaged in such processed PVC. Further, processing of PVC with such additives, almost always conducted in the presence of one or more thermal stabilizers for PVC, is improved by removal of impurities in the additives which can affect the thermal stability of the blend. For polyglutarimides, it has been discovered, as disclosed in U.S. application Ser. No. 07/638,585, now abandoned filed on the same day as the present invention and with a common assignee, that lowering the volatile content, as measured by lowering of residual amine, has a positive effect on stability in PVC blends. The volatiles affecting stability may be residual monomethylamine, dimethylamine or trimethylamine formed as by-products of the imide formation, or other volatile materials formed during the imidization process. A decrease below 30 ppm of such residual amines is desirable to improve thermal stability.
There has thus been sought a practical solution to improving both the food-simulant and thermal stability aspects of PVC//polyglutarimide blends at little additional expense to the cost of the blend.